Changes:
* Added functionality for widget composition. This allows to gather
update information from nested widgets and gradually combine it to the
facet level.
* Details' facet update method was split to several parts to allow finer
overriding in subclasses.
* Sudo rule and HBAC rule details facet was reworked to use new
functionality.
Changes in composite_widget (formal details_facet):
* added get_update_info method (for all widgets)
* modified save method
* added links to parent widget and facet
Note: cleanup of sudo options section may be part of
https://fedorahosted.org/freeipa/ticket/1690

Some issues:

1. The IPA.hbacrule_details_facet uses IPA.sudo.enable_widget(). This
makes HBAC unnecessarily dependent on sudo. The enable_widget should be
moved to widget.js or rule.js which is shared between HBAC and sudo.

2. The set_facet() is added to widget. I don't think we want to make
widget dependent on facet. The facet so far is only used by
IPA.sudo.enable_widget. In IPA.sudo.options_section the facet object is
passed as a parameter in spec.

3. When updating sudo rule, in the original code the rule is
enabled/disabled after all the modifications are done. In the new code
it's reversed. I'm not sure the importance of the execution order for
this particular situation, but suppose it is, is there a way to maintain
the original order?

4. The IPA.header_widget is not really a widget, it's actually part of
layout. In the current implementation a widget always corresponds to an
attribute, so it will be loaded, saved, etc. Since there is no attribute
name matching the header name currently this is not a problem, but it
can pollute the namespace.

5. In details facet update(), instead of storing the callbacks in
update_custom_on_win and update_custom_on_fail and requiring the
subclasses to call them, it might be better to call them from the
update() itself:

8. The IPA.sudo.rule_details_runas_widget uses composite widget instead
of section. They are right now functionally identical, but I suppose the
composite widget is an abstract class and the section will later contain
code specific to section.

9. It's probably better to define the proper update_info and field_info
classes rather than constructing them on the fly.